1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a driving method thereof adapted to reduce the generation of heat and power consumption of a data driving circuit and to prevent the deterioration of the picture quality in the data of weakness patterns.
2. Discussion of the Related Art
A liquid crystal display displays images by controlling the light transmittance of liquid crystal cells in response to a video signal. A liquid crystal display of an active matrix type actively controls data by switching a data voltage applied to the liquid crystal cells using a thin film transistor (TFT) formed at every liquid crystal cell Clc, as illustrated in FIG. 1, thereby improving the picture quality of a motion image. As shown in FIG. 1, reference label “Cst” denotes a storage capacitor for sustaining the data voltage charged to the liquid crystal cell “Clc,” “D1” denotes a data line through which the data voltage is supplied, and “G1” denotes a gate line through which a scan voltage is supplied.
The liquid crystal display is driven according to an inversion method in which a polarity is inverted between neighboring liquid crystal cells. The polarity is inverted whenever a frame period is shifted in order to reduce a direct current (DC) offset component and the degradation of liquid crystals. However, the swing width of the data voltage, which is supplied to the data lines whenever the polarity of the data voltage is shifted, is increased, thereby generating a great amount of current in a data driving circuit. Thus, problems of rising temperature due to increase in heat generation and power consumption of the data driving circuit increases sharply.
In order to reduce the swing width of the data voltage supplied to the data lines, thereby reducing the heat generated temperature and power consumption of the data driving circuit, a charge sharing circuit or a precharge circuit is adopted in the data driving circuit. However, the effects of these circuits do not provide a satisfactory result.
Further, if the polarity of the data voltage is driven according the inversion method, the charging amount of a liquid crystal cell charged by the data voltage of a positive polarity is different from that of a liquid crystal cell charged by the data voltage of a negative polarity. Thus, there is a problem in that the picture quality is degraded.
For example, as shown in FIG. 2, assuming that a liquid crystal cell is charged by the data voltage of a positive polarity and then by the data voltage of a negative polarity for representing the same gray level as that of the data voltage of the positive polarity, the liquid crystal cell maintains a voltage Vp(+) whose absolute value voltage may be lowered by as much as ΔVp due to parasitic capacitance of the TFT after being charged by the data voltage of the positive polarity. Then, the liquid crystal cell maintains voltage Vp(−) whose absolute value voltage may be increased by as much as ΔVp due to parasitic capacitance of the TFT after being charged by the data voltage of the negative polarity.
Accordingly, a liquid crystal cell of a normally black mode liquid crystal display has light transmitted therethrough with a higher light transmittance when being charged by the data voltage of a negative polarity for representing the same gray level as that of the data voltage of a positive polarity than that of the data voltage of the positive polarity. In the normally black mode, the higher the voltage charged in a liquid crystal cell, the higher the light transmittance of the liquid crystal cell.
Further, a liquid crystal cell of a normally white mode liquid crystal display has light transmitted therethrough with a lower light transmittance when being charged by the data voltage of a negative polarity for representing the same gray level as that of the data voltage of a positive polarity than that of the data voltage of the positive polarity. In the normally white mode, the higher the voltage charged in a liquid crystal cell, the lower the light transmittance of the liquid crystal cell.
In addition, a liquid crystal display has a low picture quality in the data pattern of a specific picture according to a correlation between the polarity pattern of a data voltage applied to the liquid crystal cells and the gray levels of data. Representative factors that degrade the picture quality include a phenomenon in which a greenish tint is generated in a display screen, and flicker is generated in which the luminance of a screen is shifted periodically.
For example, greenish tint may be generated in a display image when a liquid crystal display is driven according a vertical 2-dot and horizontal 1-dot inversion method (V2H1) in which the polarity of a data voltage applied to the liquid crystal cells every vertical 2-dot (or 2 liquid crystal cells) is inverted, and the polarity of a data voltage applied to liquid crystal cells every horizontal 1-dot (or 1 liquid crystal cell) is inverted. In addition, the gray levels of data supplied to odd pixels are white gray levels and the gray levels of data supplied to even pixels are black gray levels within a 1 frame period, as shown in FIG. 3. In other words, in the first, second, fifth, and sixth lines L1, L2, L5, and L6, the data voltage of all green (G) data, which have the greatest influence on the luminance, of red (R), green (G), and blue (B) data, have a negative polarity. Therefore, greenish tint is generated in the first, second, fifth, and sixth lines L1, L2, L5, and L6. This greenish phenomenon is generated because the green (G) data is biased toward any one polarity.
Another example of this greenish phenomenon is shown in FIG. 4. As shown in FIG. 4, greenish tint is generated in a display image when a liquid crystal display is driven according to a vertical 2-dot and horizontal 1-dot inversion method (V2H1), and the gray levels of data supplied to odd subpixels are white gray levels and the gray levels of data supplied to even subpixels are black gray levels.
When a liquid crystal display is driven according to a vertical 1-dot and horizontal 1-dot inversion method (V1H1) in which the polarity of a data voltage is inverted every vertical 1-dot and horizontal 1-dot so that the polarities of data voltages applied to adjacent liquid crystal cells in vertical and horizontal directions are inverted. For the data voltages that include a data voltage of white gray level and a data voltage of black gray level alternately disposed every 1 subpixel within a one frame period as shown in FIG. 5, a flicker phenomenon in which the luminance of a display image is shifted every frame period is generated. In other words, all the data voltages of white gray levels have a positive polarity and all the data voltages of white gray levels in a next frame have a positive polarity within 1 frame period. Consequently, the luminance of a display image is shifted every frame period causing flicker.